Counterfeiting documents such as banknotes is becoming now more than ever a serious problem, due to the availability of high-quality and low-priced color photocopiers and desk-top publishing systems (see, for example, "Making Money", by Gary Stix, Scientific American, March 1994, pp. 81-83).
The present invention is concerned with providing a novel security element and authentication means of enhanced security for banknotes, cheques, credit cards, travel documents and the like, which is even more difficult to counterfeit than present banknotes and security documents.
Various sophisticated means have been introduced in prior art for counterfeit prevention and for authentication of documents. These include the use of special paper, special inks, watermarks, micro-letters, security threads, holograms, etc. Nevertheless, there is still an urgent need to introduce further security elements, which do not considerably increase the cost of the produced documents.
Moire effects have already been used in prior art for the authentication of documents. For example, United Kingdom Pat. No. 1,138,011 (Canadian Bank Note Company) discloses a method which relates to printing on the original document special elements which when counterfeited by means of halftone reproduction show a moire pattern of high contrast. Similar methods are also applied to the prevention of digital photocopying or digital scanning of documents. In all these cases, the presence of moire patterns indicates that the document in question is counterfeit. However, in prior art no advantage is taken of the intentional generation of a moire pattern having a particular intensity profile, whose existance, and whose precise shape, are used as a means of authentication of the document. The approach on which the present invention is based further differs from that of prior art in that it not only provides fulll mastering of the qualitative geometric properties of the generated moire (such as its period and its orientation), but it also permits to determine quantitatively the intensity levels of the generated moire.